Nonvolatile memory device

ABSTRACT

The gate structure for a nonvolatile memory device comprising an EEPROM and a latch transistor is fabricated on a substrate by patterning the EEPROM&#39;s floating gate in a first polysilicon layer, patterning the EEPROM&#39;s control gate over the floating gate in a second polysilicon layer, and then collectively patterning the second and first layers to form the latch transistor&#39;s stacked gate. The stacked gate includes a thin gate that is electrically connected to the EEPROM floating gate and a protective layer over and electrically isolated from the thin gate. The stacked gate design eliminates unwanted polysilicon spacers between the latch transistor&#39;s channel and its drain and source regions, which improves the control of the memory device. The protective layer prevents ion penetration during the implantation of the latch transistor&#39;s drain and source regions. The fabrication process and thinness of the latch transistor gate improve the linewidth control of other transistors formed on the substrate and the latch transistor by avoiding overetching and reducing the normal etching time for the latch gate, respectively.

This is a divisional of application Ser. No. 08/554,220, filed Nov. 6,1995, now U.S. Pat. No. 5,578,515.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a nonvolatile memory deviceand a method for fabricating its gate structure, and more specificallyto a nonvolatile memory device having an ElectricallyErasable-Programmable Read-Only Memory (EEPROM) and a stacked gate latchtransistor, and a method for fabricating the EEPROM and latch transistorgate structure.

2. Description of the Related Art

U.S. Pat. No. 4,571,704, "Nonvolatile Latch" to F. Bohac and assigned toHughes Aircraft Company, the assignee of the present invention,discloses a bistable latch circuit which can be electrically programmedto be stable in only one of its two states so that this same state isalways set when power is applied to the circuit. The purpose of thenonvolatile latch is to store data, and to retain that data if power isinterrupted.

Since the circuitry of the Bohac patent is also applicable in general tothe present invention, it will be described in some detail. FIG. 1 is aschematic diagram of the nonvolatile latch 10 disclosed in the Bohacpatent comprising four interconnected MOS transistors 12, 14, 16 and 18,and two EEPROMs 20 and 22. Transistors 12 and 14 are CMOS P-channelfield effect transistors (FETs) whose sources 24, 26 are coupledtogether at a node 28. The gate 30 of transistor 12 is coupled to thedrain 32 of transistor 14 at a node 34. Similarly, the gate 36 oftransistor 14 is coupled to the drain 38 of transistor 12 at a node 40.

Transistors 16, 18 are N-channel latch MOSFETs whose drains 42, 44 areconnected to the drains 38, 32 of the P-channel transistors 12, 14 andwhose sources 46, 48 are connected to a low supply voltage V_(S),typically ground. EEPROMs 20, 22 comprise p-well tie downs 50, 52,floating gates 54, 56 and control gates 58, 60. The floating gates 54,56 are electrically connected to the gates 62, 64 of latch transistors16, 18. The EEPROM control gates 58, 60 are connected to the opposingEEPROM p-well tie downs 52, 50 at nodes 66, 68, respectively.

Binary DATA and PROGRAMMING voltages 70, 72 are applied to a programmingcircuit 74. The PROGRAMMING voltage is also applied to node 28. Theprogramming circuit outputs voltage signals to nodes 66, 68 inaccordance with the DATA and PROGRAMMING voltages. When the PROGRAMMINGvoltage is low, the DATA voltage establishes the binary data value to bestored by the latch. When the PROGRAMMING voltage is high, the storeddata is read out of the latch at node 34. The inverse of the stored datacan be read out at node 40.

A typical nonvolatile memory could have several thousand nonvolatilelatch cells, such as the cell shown in FIG. 1, for storing data. Thememory would also include support circuitry for addressing theindividual cells, programming the cells and reading out the data. Thefabrication process currently used by Hughes Aircraft Company for thenonvolatile latch 10, and specifically for the gate structure, isdescribed in conjunction with FIGS. 2a-2d (which are not to scale) forthe EEPROM 20, its latch transistor 16 and P-chann el transistor 12. Thefabrication of the P-channel transistor and the problems in controllingits linewidth are common to other P and N-channel transistors on thesubstrate.

As shown in FIG. 2a, a semiconductor substrate 80 has P-wells 82, 83that are separated by field oxide regions 84 to form active areas forthe EEPROM 20 and the latch transistor 16, respectively. The P-channeltransistor is formed directly in the N-substrate. An oxide layer 86 isformed over the substrate and an electron tunnel 88 is etched in theoxide layer above the EEPROM active area 82. A polysilicon layer (notshown) having a thickness of approximately 5000Å is deposited over thesubstrate, patterned with a photoresist and etched to form the EEPROMfloating gate 54 and the latch transistor gate 62 over their respectiveactive areas 82 and 83. A dielectric oxide layer 90 is then formed onthe top and sides of the EEPROM floating gate 54 and latch transistorgate 62.

To form the EEPROM control gate 58 and the P-channel transistor gate 30,a second polysilicon layer 92 is deposited over the substrate as shownin FIG. 2b. The layer 92 Generally has a thickness of approximately5,000Å. However, the deposition process produces a smooth polysiliconlayer whose height from the substrate surface decreases smoothly fromapproximately 10,000Å above the EEPROM floating gate 54 and the latchtransistor Gate 62 (the combined thicknesses of the two polysiliconlayers) to approximately 5,000Å along the substrate surface. Therefore,the thickness of the deposited polysilicon near either side of the latchtransistor gate 62 and EEPROM floating Gate 54 is greater than 5,000Å. Aphotoresist pattern 94 is patterned over the second polysilicon layer todelineate the EEPROM control gate 58 and the P-chann el transistor gate30. The exposed polysilicon is etched to a depth of approximately 5000Åto form the EEPROM control gate 58 and the P-channel gate 30 as shown inFIG. 2c.

If the etching process were stopped at this point, polysilicon spacers96 would be left on either side of the latch transistor gate 62 due tothe increased thickness of the second polysilicon layer in theseregions. The spacers are approximately 3,000Å wide. The EEPROM controlgate 58 covers the floating gate 54, and hence no spacers are formedaround the EEPROM.

When the drain and source regions of the latch transistor aresubsequently ion implanted, the spacers would block the ions frompenetrating the underlying substrate, leaving gaps between a channelarea under the latch transistor gate 62 and drain and source regions oneither side of the channel. The gaps would seriously increase thechannel resistance, thus slowing the latch transistor.

Therefore, as shown in FIG. 2d, the second polysilicon layer istypically overetched, i.e., the etching time is extended beyond thenormal time for a given layer thickness, to remove the polysiliconspacers 96, after which the remaining photoresist is dissolved. Thenormal etching process undercuts the P-channel transistor gate 30 andreduces its width by approximately 0.1μ for each 1μ of vertical etch.Overetching substantially increases the amount of undercutting. Forexample, a 3μ wide P-channel gate 30 may be undercut so that its actuallinewidth is approximately 2.2μ.

The extent of the undercut is not known prior to etching and will varyfrom device-to-device, making it very difficult to compensate for theundercut in the original design or to know precisely what the actualgate width is after etching. The poor linewidth control reduces theeffective length of the channel, thus changing the performance of theP-channel transistor. If the channel length becomes too short,source-to-drain punchthrough can result, destroying the transistor.Consequently, the P-channel transistor is limited to gate widths greaterthan 1μ.

A common practice to diminish the undercutting problem is to reduce thethickness of the dielectric oxide layer 90 and the first polysiliconlayer. This reduces the overetching time, which in turn reduces theamount of undercutting. However, a thin dielectric layer reduces thebreakdown voltage between the EEPROM floating gate and its control gate.A thin first polysilicon layer will allow ion implantation penetrationin the latch transistor's channel during the source/drain implantationswhich can increase the leakage current in the device.

SUMMARY OF THE INVENTION

The present invention seeks to provide a nonvolatile memory device and amethod for fabricating its gate structure with improved linewidthcontrol, a thinner latch gate and submicron gate widths.

This is accomplished with an EEPROM and a stacked gate latch transistorhaving a shared gate structure. The shared gate structure is fabricatedby depositing a first polysilicon layer over a semiconductor substratehaving EEPROM and latch transistor active areas and an electron tunnel.The polysilicon layer is patterned to form an EEPROM floating gate abovethe electron tunnel, and a first latch transistor gate region that liesover its active area and is electrically connected to the floating gate.A dielectric oxide layer is formed over the floating gate and the firstlatch transistor gate region, and a second polysilicon layer isdeposited over the EEPROM floating gate and first latch transistor gateregion on the substrate.

The second polysilicon layer is patterned to form an EEPROM control gateover the floating gate, and a second latch transistor gate region overthe first latch transistor gate region. The first and second layers arethen patterned with one additional masking step to form a latchtransistor stacked gate comprising a protective layer and a gate etchedfrom the second and first latch transistor gate regions, respectively.The latch transistor gate and EEPROM floating gate are electricallyconnected. The second polysilicon is self-aligned to the firstpolysilicon in the latch transistor. Therefore, there are no polysiliconspacers, and hence no additional resistance between the channel regionand its drain and source regions, which can affect device performance.

By forming the protective layer, which blocks ion penetration duringdrain/source implantation, over the latch gate, the gate can be madethinner, which makes it easier to do submicron processing. The linewidthcontrol of other transistors formed by the second polysilicon is alsoimproved by avoiding overetching. By enhancing linewidth control,performance of the transistors can be more accurately estimated and thetransistors can be fabricated with submicron gate widths.

For a better understanding of the invention, and to show how the samemay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, described above, is a schematic diagram of a known nonvolatilelatch;

FIGS. 2a-2d, described above, are sectional views of a portion of thenonvolatile latch shown in FIG. 1 that illustrate a known gate structurefabrication process;

FIG. 3 is a plan view of a portion of a nonvolatile latch; and

FIGS. 4a-4k, 5a-5k and 6a-6k are sectional views illustrating thefabrication of the EEPROM, latch transistor, and P-channel transistorgate structures, respectively, for the nonvolatile latch shown in FIG.3.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a process for fabricating the nonvolatile latch,described above, and specifically the gate structure for the EEPROM andlatch transistor. The fabrication process produces a stacked gate forthe latch transistor instead of the conventional single polysiliconlayer gate described above. The stacked gate includes a thin polysilicongate, a dielectric layer and a protective electrically isolatedpolysilicon layer. The fabrication process and the thin polysilicon gateimprove the linewidth control of the P-channel and support circuitrytransistors and the latch transistor by avoiding overetching andreducing the normal etching time for the latch gate, respectively. Byimproving linewidth control, the transistor performance is improved andthe transistors can be fabricated with submicron gate widths, thusreducing chip sizes.

FIG. 3 shows one of the two symmetrical branches 100 of a nonvolatilelatch which, except for the features of the invention, is generallysimilar to the circuit of FIG. 1. The other branch is fabricated in thesame manner and at the same time as the branch shown in FIG. 3, and isnot shown for purposes of simplification. The branch includes an EEPROM102, a latch transistor 104 and a P-channel transistor 106 which arefabricated on a semiconductor substrate 108. Active areas 110, 112 and114 are defined in the substrate 108 by field oxide regions for theEEPROM, latch transistor and P-channel transistor, respectively.

The gate structure is preferably fabricated by three successivepatterning steps, each step comprising forming a photoresist mask,etching the exposed polysilicon and then dissolving the photoresist. Afirst polysilicon (POLY1) layer, a second polysilicon (POLY2) layer andboth POLY2 and POLY1 layers are patterned to form the gate structure byetching the polysilicon exposed by POLY1, POLY2 and Stacked Gate POLY(SGPO) masks, respectively. Etching the polysilicon exposed by the POLY1and SGPO masks forms the EEPROM floating gate 116 and the latchtransistor stacked gate 118, which are electrically connected to eachother. The stacked gate comprises a latch gate (not shown) formed fromthe POLY1 layer and a protective layer 122 that is formed above thelatch gate from the POLY2 layer. The protective layer is electricallyisolated from the latch gate, and preferably from the remainingcircuitry on the substrate; its only function being to prevent ionimplantation penetration through the latch into the substrate. Theprotective layer is closely aligned with the latch gate by etching thepolysilicon exposed by the SGPO in the second and then the firstpolysilicon layers. The EEPROM control gate 124 above the EEPROMfloating gate and the P-channel gate 126 are formed by etching thepolysilicon in the second layer that is exposed by the POLY2 mask.

Once the gate structure is fabricated, drain regions 132, 130 and sourceregions 128, 134 of transistors 104 and 106 are ion implanted on eitherside of the latch transistor stacked gate 118 and P-channel gate 126 intheir active areas, and the EEPROM well contact 136 is ion implanted inits p-well near the floating gate. The self-alignment feature of thestacked gate prevents ion penetration of the latch gate and ensures thatgaps will not be formed between the drain and source regions and thechannel beneath the latch gate.

Metal contacts 138, 140 conduct external programming voltages to thewell contact 136 and EEPROM control gate 124. Metal contact 142 connectslatch transistor source 128 to a low supply voltage, typically ground.The latch transistor drain 132 and the P-channel drain 130 are connectedby contact 144. The P-channel source 134 is connected to a programmingvoltage through metal contact 146. Contact 148 connects the P-channelgate 126 to the P-channel drain in the other branch of the nonvolatilelatch (not shown).

A preferred fabrication is illustrated in FIGS. 4a-4k, 5a-5k and 6a-6k,which are respectively sectional views of the EEPROM 102, latchtransistor 104 and P-channel transistor 106 shown in FIG. 3 insuccessive stages of fabrication. The P-channel transistor is describedfor the purposes of simplicity and clarity. When an entire chip isfabricated, including perhaps 32,000 nonvolatile cells and their supportcircuitry, other P and N-channel transistors are formed simultaneouslywith P-channel transistor 106 and realize the same improvement inlinewidth control. Conventional semiconductor processing techniques areused to prepare the substrate prior to fabricating the gate structureand to complete the nonvolatile memory device after gate fabrication.

FIGS. 4a, 5a and 6a are sectional views of the EEPROM, latch transistorand P-channel transistor areas of the substrate 108, respectively. Inthe preferred embodiment, the EEPROM and latch transistor are N-channeldevices. The invention is also applicable to P-channel devices, althoughthey require more power. The N-silicon substrate 108 has P-wells 150a,150b that are separated by field oxide regions 152, approximately 7,000Åto 10,000Å thick, to form the active areas 154a, 154b for the EEPROM andlatch transistor, respectively. The active area 154c for the P-channeltransistor is formed in the substrate 108. A thin oxide layer 156,approximately 400-500Å thick, is formed over the substrate, and anelectron tunnel 158, suitably 0.5-2μ wide, is etched in the oxide layer156 above the EEPROM active area 154a. A thin tunnel oxide 160,approximately 80-90Å thick, is formed over the EEPROM area including theelectron tunnel 158.

In FIGS. 4b, 5b and 6b, a polysilicon (POLY1) layer 162 is formed on topof the substrate 108. The POLY1 layer must be thick enough to formtransistor gates, approximately 1500Å thick, but does not have to bethick enough to prevent ion implant penetration. The POLY1 layersuitably has a resistance of approximately 200 ohms per square. Thesquare unit is defined by the minimum linewidth of the particularfabrication process. A dielectric layer 168, equivalent to about 325Å ofsilicon oxide dielectric characteristics, is also formed over POLY1. Thedielectric layer 168 is suitably a silicon oxide/nitride/oxynitridecomposite.

In FIGS. 4c, 5c and 6c, a POLY1 photoresist 164 is patterned over thePOLY1 layer 162 and the dielectric layer 168, and delineates the EEPROMfloating gate 116, covers the latch transistor active area 154b andexposes the POLY1 layer 162 over the P-channel active area 154c. Thecomposite dielectric layer 168 and the POLY1 layer are etched to formthe EEPROM floating gate 116 and a latch transistor gate region 166,after which the remaining photoresist is dissolved as shown in FIGS. 4d,5d and 6d. The remaining thin oxides 156 and 160 are also removed.Another dielectric oxide layer 169, approximately 450Å thick is formedover the substrate to provide the gate oxide for the P-channeltransistor. The same dielectric layer 169 becomes thicker, approximately1000Å, and adheres to the sides of the EEPROM floating gate 116 and thelatch transistor gate region 166, as shown in FIGS. 4e, 5e and 6e.

As shown in FIGS. 4f, 5f and 6f, a polysilicon (POLY2) layer 170 isformed over the EEPROM floating gate 116, latch transistor gate region166 and P-channel area on the substrate 108. The POLY2 layer should bethick enough, approximately 5,000Å thick, to prevent ions during the ionimplantation of the drain/source regions from penetrating through to thesubstrate. The POLY2 layer 170 suitably has a resistance ofapproximately 15 ohms per square. In FIGS. 4g, 5g and 6g, a POLY2photoresist pattern 172 is patterned over the POLY2 layer 170 anddelineates the EEPROM control gate 124, covers the latch transistor gateregion 166 and delineates the P-channel transistor gate 126. The exposedportions of the POLY2 layer are etched and the photoresist is dissolvedto form the EEPROM control gate 124, a latch transistor gate region 174and the P-channel gate 126 as shown in FIGS. 4h, 5h and 6h.

A SGPO photoresist 176 is patterned over the substrate to cover theEEPROM 102 and the P-channel transistor 106, and to delineate the latchtransistor stacked gate 118 over the latch transistor gate regions 166and 174 as shown in FIGS. 4i, 5i and 6i. As shown in FIGS. 4j, 5j and6j, the exposed portions of the POLY2 and POLY1 layers are etched toform the stacked gate 118, after which the SGPO photoresist isdissolved. The stacked gate comprises the protective POLY2 layer 122that is positioned over and separated from a gate 180 (remaining fromthe original gate region 166) by the remaining portion of the dielectriccomposite layer 168, which forms a dielectric spacer 182. The etching isaccomplished in three steps: the POLY2 layer 170 is etched to a depth of5,000Å, the exposed dielectric oxide layer 168 is removed, and the POLY1layer 162 is etched to a depth of 1,500Å.

As shown in FIGS. 4k, 5k and 6k, the N⁺ drain and source regions 132,130 and 128, 134 of latch transistor 104 and P-channel transistor 106,and the N⁺ p-well tie down 136 of the EEPROM 102, are ion implanted intothe substrate 108. The active areas 154b and 154c form channels for thelatch and P-channel transistors beneath their respective gates andbetween their drain and source regions. The protective POLY2 layer 122over the thinner latch transistor gate 180 prevents ions frompenetrating through the stacked gate to the transistor's channel 184.The protective layer is preferably electrically isolated from the latchgate and the remaining circuitry on the substrate.

The fabrication process does not form polysilicon spacers, and thereforedoes not create gaps between the latch transistor channel 184 and itsdrain and source regions 128 and 132. Furthermore, the process does notrequire overetching when the P-channel gate is formed, and hence thelinewidth of the P-channel transistor 106 is approximately equal to itsdesigned linewidth. For example, a P-channel gate 106 that was designedto have a width of 3μ was fabricated with an actual width ofapproximately 2.9μ, whereas in the prior art the gate was undercut toapproximately 2.2μ. The protective layer 122, which prevents ionimplantation penetration, allows the latch gate 180 and POLY1 layer 162from which the latch gate is formed to be thin. Without the protectivelayer, the latch gate would have to block ion penetration. By thinningthe POLY1 layer from approximately 5,000Å to 1,500Å, the etching timefor the latch gate is reduced which diminishes the undercutting of thegate and improves the linewidth precision. By improving the linewidthcontrol, the transistors' channel widths are more tightly controlled,improving performance and increasing yield rates. Furthermore, submicron(less than 1 micron) gates for the P-channel and support circuitrytransistors can be designed and fabricated, thus greatly increasing thenumber of devices that can be fabricated on a given substrate.

While one embodiment of the invention has been shown and described,numerous variations and alternate embodiments will occur to thoseskilled in the art. For example, the fabrication process is not limitedto the nonvolatile latch circuit. The fabrication of the EEPROM andlatch transistor gate structure could be used in other memory devices.Such variations and alternate embodiments are contemplated, and can bemade without departing from the spirit and scope of the invention asdefined in the appended claims.

We claim:
 1. A nonvolatile memory device, comprising:a semiconductorsubstrate having Electrically Erasable-Programmable Read-Only Memory(EEPROM) and latch transistor active areas; an electron tunnel that isdisposed above said EEPROM active area; an EEPROM having a floating gatethat is positioned over said electron tunnel and a control gate that ispositioned over said floating gate, said control gate being capable ofreceiving a data signal and storing it in said floating gate; and alatch transistor having a stacked gate that is positioned over saidlatch transistor active area, said stacked gate comprising a latch gateand a polysilicon layer above said latch gate, said latch gate beingelectrically connected to said floating gate, and said latch transistorbeing capable of receiving a programming signal to read out the storeddata signal.
 2. A nonvolatile memory device in accordance with claim 1,wherein said polysilicon layer is electrically isolated from said latchtransistor gate.
 3. A nonvolatile memory device in accordance with claim1, wherein drain and source regions are ion implanted in said substrateon either side of said latch transistor gate, said protective layerbeing thick enough to prevent ion implantation penetration of saidsubstrate and said latch transistor gate being thinner than the latchgate thickness that would be necessary to prevent ion implantationpenetration in the absence of said protective layer.
 4. A nonvolatilememory device in accordance with claim 1, wherein said said latch gatedefines a channel in said substrate, and drain and source regions areion implanted in said substrate on either side of said latch transistorgate such that they are immediately adjacent said channel to reduce theresistance between the drain and source regions and said channel.